The study of the properties of the hydroxyapatite/electrolyte interface

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1 A N N A L E S U N I V E R S I T A T I S M A R I A E C U R I E - S K Ł O D O W S K A L U B L I N P O L O N I A VOL. LXIV, 2 SECTIO AA 2009 The study of the properties of the hydroxyapatite/electrolyte interface W. Janusz and E. Skwarek Faculty of Chemistry, Maria Curie-Sklodowska University, pl. Marii Curie-Skłodowskiej 3, Lublin, Poland, 1ewunias@hermes.umcs.lublin.pl A physicochemical property of hydroxyapatite (HAP) [Ca 10 (OH) 2 (PO 4 ) 6 ] surface was studied and parameters of electrical double layer of the hydroxyapatite/nacl solution system were described. The study sample of hydroxyapatite was synthesized. The structure of the hydroxyapatite was confirmed using X-ray diffraction (XRD), its particle size by PCS (photon correlation spectroscopy) AFM and SCM microscope and its surface properties by means of adsorption-desorption of nitrogen (ASAP), Raman spectra,. Then the point of zero charge and the isoelectirc point were measured. 1. INTRODUCTION Hydroxyapatite [Ca 10 (OH) 2 (PO 4 ) 6 ] is the structural prototype for the main inorganic constituent of bone and teeth, is also one of the principal minerals in commercial phosphate ores. The surface chemistry characteristics of hydroxyapatite is important in understanding the growth, dissolution and adhesion mechanisms of bone and teeth tissues and in enrichment of minerals such as flotation and flocculation [1,2,3] Methods of hydroxyapatite preparation may have influence on specific chemical properties of adsorbent surface. The high purity, high density and desirable microstructure ceramic hydroxyapatite has been prepared using processes: microemulsion synthesis, sol-gel, co-precipitation or hydrothermal This article is dedicated to Professor Roman Leboda on the occasion of his 65 th birthday

2 12 W. Janusz and E. Skwarek reactions [1]. The study of the electrical double layer structure for the hydroxyapatite/electrolyte solution interface reveal that the ph pzc and ph iep data lie in a wide range, from 4.35 for dental HAP up to 8.6 for synthetic samples, and it is very difficult to compare them. The main reason for this is the fact that all experimental results were obtained for HAP samples of different origin, stoichiometry and purity, under various experimental conditions, and by different experimental techniques. However the solubility of HAPs in determination of surface charge density and ph pzc by means of potentiometer titration should be taken into account. The studies of the electrical double layer at the hydroxyapatite/electrolyte solution interface shows that besides Ca 2+ and phosphate, the H + and OH - are potential determining ions[4,5]. The charge on hydroxyapatite arises as a result of the various reactions of surface hydroxyl groups and unequal adsorption from solution of ions of opposite charge [6,7]. Because hydroxyapatite is a basic salt, there was two types of groups on the surface: hydroxyl and phosphatic. It is no doubt that the phosphatic groups have more acid character that hydroxyl one, those has alkaline character. The reactions of surface groups at the hydoxyapatite that create surface charge are following: H CaOH 2 CaOH (1) + CaOH CaO + H (2) + PO4 H PO4 + H (3) PO 4 + Ca PO4 + Ca (4) The goal of this paper is characterization of hydroxyapatite prepared, determination of ph pzc and ph iep points, establishing of zeta potential on ph dependence and determination of calcium ions dependence on ph and electrolyte concentration. 2. EXPERIMENTAL The sample of hydroxyapatite have been synthesized on the basis of prescription described in the literature [2]. The synthesis reaction occurs as follows: 4 Ca( OH ) 2 + 6CaHPO4 2H 2O Ca10 ( PO4 ) 6( OH ) H 2O (5)

3 The study of the properties of the hydroxyapatite/electrolyte interface 13 A aqueous solutions of reagents was prepared by dissolution of CaHPO 4 2H 2 O ( g) and Ca(OH) 2 ( g) in 40 ml deionized water respectively. For ph=9, the initial solution was adjusted by adding CH 3 COOH solution. The ready-adjusted solution was put in a teflon vessel and heated at 120 o C for 24h, and then cooled to a room temperature in natural way. Next the precipitate was filtered and washed with deionized water. Finally, the solid was dried at 60 o 100 o C. To remove ionic type contaminations, which might influence the measurements, the hydroxyapatite was washed with double distilled water until constant conductivity about 0.05 µs/cm was achieved. The prepared hydroxyapatite sample has been characterized using X-ray diffraction (XRD), adsorption-desorption of nitrogen (ASAP), PCS (photon correlation spectroscopy), Raman spectra, AFM and scanning microscope. Zeta potential was measured in the NaCl as a background electrolyte in three concentrations as a function of ph. Because for the studied system κa>>1 the zeta potentials were calculated using the Smoluchowski equation. Calcium ions concentration has been measured by ionselective electrode. Then the point of zero charge and the isoelectric point were measured. A crystallographic structure of samples was confirmed by XRD using DRON-3 diffractometr using CuK α radiation and nickel filters. 3. RESULTS AND DISCUSSION Figure 1 shows the peaks of X-ray diffraction patterns. We observed peak characteristic for hydroxyapatite: 25,9 35%; 31,75 100%; 32;96 55%; 39,84 20%; 46,7 40%; 49,5 30%. Between them it can be seen peaks characteristic for calcium carbonate (2θ = 29,4 100% and %). Calcium carbonate was formed probably during washing out of the sample with distilled water [8]. The specific surface of the hydroxyapatite samples was determined by adsorption-desorption of nitrogen using ASAP 2405 (Accelerated Surface Area and Porosimetry, Micromeritics Instruments, Co.). Table 1 present structural parameters of hydroxyapatite. The particle size and polidispersity coefficient has been determined by PCS (Photon Correlation Spectroscopy) using Zetasizer 3000 by Malvern. The average size of polidispersic samples was 420 nm. Figure 3 shows Raman spectra. The one band in the spectra may be suspect of the presence of carbonate ions in the studied sample[9,10]. Most bands at the Figure 2 are characteristic for pure hydroxyapatite but only band at cm -1 may be connected with ν 3 (PO 4 ), as well as with carbonates presence in the sample.

4 14 W. Janusz and E. Skwarek Fig. 1. Powder X- diffraction pattern. Tab. 1. Selected structural parameters. BET surface area [m2/g] Langmuir surface area [m2/g] BJH cumulative adsorption surface area of pores between 1.7 and 300 nm diameter [cm3/g] BJH cumulative desorption surface area of pores between 1.7 and 300 nm diameter [cm3/g] Average pore diameter (4 V/A by BET) [nm] BJH adsorption on average pore diameter (4 V/A) [nm] BJH desorption on average pore diameter (4 V/A) [nm] 8.39

5 The study of the properties of the hydroxyapatite/electrolyte interface PO 4 ν Intensity PO 4 ν 2 PO 4 ν PO 4 ν 2 CO 2 ν Raman shift [cm -1 ] Fig. 2. Raman spectra. Figure 3 shows AFM micrographs of several hydroxyapatite samples. It shows formation of monodisperse change crystals. Fig. 3. AFM micrograph of hydroxyapatite.

6 16 W. Janusz and E. Skwarek Photo at Figure 4 made by scanning microscope, shows hydroxyapatite particles dimensions and surface structure. As can be seen, besides large particles about 20 µm thata are aggregates, also smaller of 1 µm are visible. Because crystallite dimension calculated from XRD by means Scherrer method is 36nm, it leads to conclusion that hydroxyapatite particles observed in the Figure 4 are aggregates that are huge amount of small crystals that together forms porous structure. Fig. 4. Scanning microscopy. I. D. Smičiklas et al. has given a a copmarison of ph pzc and ph iep values for hydroxyapatite that are obtained for different basic electrolytes using potentiometric titration and electrophoresis [11]. There are some samples with

7 The study of the properties of the hydroxyapatite/electrolyte interface 17 the ph iep < 5. L. C. Bell et. al. has also presented a juxtaposition of ph pzc values [12]. They are ranged from 4.5 to 7 for different electrolytes. L. M. Rodriguez-Lorenco et. al. have obtained a typical dependence zeta potential as a function of ph and ph iep is at ph=7.3. They have investigated at the hydroxyapatite/kcl interface in ph range from 4.5 to 12 [3]. Due to process of dissolution, various weights of hydroxyapatite for potentiometric titration were taken with the purpose of determining ph pzc Figure Surface charge density [ µc/cm 2 ] 0-20 NaCl Concentr. [mol/dm 3 ] ph Fig. 5. Surface charge density at the hydroxyapatite / M NaCl solution interface as a function of ph. The chart with dependence of calcium ions concentration on ph in hydroxyapatite/nacl system is shown at Figure 6. Change of background electrolyte concentration from to 0.1 mol/dm 3 had little influence on calcium ions concentration, also is visible that for ph=10 calcium ions concentration is lower than electrode sensivity which caused zero readout. Figure 7 present the zeta potential as a function of ph for two concentrations of NaCl solution. It can be seen that increase of electrolyte concentration in the system causes the lowering of zeta potential. The point ph iep = 4.3 for sample. It can be seen also that the point ph iep is shifted towards lower values of ph. It is a result of the adsorption reactions. For background electrolyte it may occur in a porous surface that characterizes an heterogenic surface. Sample has a porous surface so that for lower electrolyte concentration double layer can be

8 18 W. Janusz and E. Skwarek overloaded. As a consequence holes can be closed during electrophoresis and surface features may not be revealed. The difference in the position of ph IEP and ph pzc may also indicate the specific anion adsorption, because ph pzc > ph IEP. Discrepancy between ph pzc and ph iep values is caused by the method of surface charge density determination: potentiometric titration uses acid-base reactions of surface groups (engaging H+ or OH- ions only) whereas dzeta potential is also dependent on this part of surface charge, which is caused by unequal adsorption or desorption of phosphate or calcium ions Concentration Ca 2+ [mol/dm 3 ] ph Fig. 6. Ca 2+ concentration at the hydroxyapatite /NaCl solution interface as a function of ph. 4.CONCLUSIONS 1. Investigations at the hydroxyapatite/electrolyte solution system are limited by dissolution of mineral and are narrowed to ph range The results of X-ray diffraction analysis, Raman spectroscopy and specific surface area obtained by AFM and BET indicate the relationship between the synthesis parameters and edl parameters. 3. The points p Hpzc and p HIEP for sample are ph pzc =6.7 and ph IEP =4.3.

9 The study of the properties of the hydroxyapatite/electrolyte interface ζ potential [mv] ph Fig. 7. The ζ potential of hydroxyapatite/nacl solution interface as a function of ph. 5. REFERENCES [1] R. E. Riman, W. L Suchanek, K. Byrappa, C. Wei Chen, P. Shuk, C. S. Oakes, Solid State Ionics (2002). [2] J. Liu, X. Ye, H. Wang, M. Zhu, B. Wang, H. Yan, Ceramics International (2003). [3] L. M. Rodriguez Lorenzo et al., Biomaterials 22, 1847 (2001). [4] G. Yin, Z. Liu, J. Zhan, F. Ding, N. Yuan, Chem. Eng. J., (2002). [5] S. Harding, N. Rashid, K. A Hing, Biomaterials (2005). [6] G. R. Wiese, R. O James, D. E. Yates, T. W. Healy, Electrochemistry of the colloid-water interface. In Electrochemistry of Colloid-Water Interface; Bockris J. O M. Ed. ; Int. Review Sci. Phys. Chem. Series 2. ; Butterworths: London; 6, 53 (1976). [7] J. Lyklema, Fundamentals of Interface and Colloid Science. Academic Press London, 1995, b) Fundamental the Electrical Double Layers in Colloidal Systems. In Colloidal Dispersions, Goodwin J. W. Ed., Special Publication No 43; Royal Soc. Chem.: London, 47 (1981). [8] R. A. Jillavenkatesa, Condrate Sr., J. Mater. Sci (1998) [9] C. Silvaa, A. S. B. Sombrab, Raman spectroscopy measurements of hydroxyapatite obtained by mechanical alloying, J. Phys. Chem. Solids (2004). [10] G. Penel, G. Leroys, C. Rey, B. Sombretd, J. P. Huvenned and E. Bres, Infrared and Raman microspectrometry study of fluor-fluor-hydroxy and hydroxy-apatite powders, J. Mater. Sci.: Materials in Medicine (1997). [11] D. Smičiklas, S. K. Milonjič, P. Pfendt, S. Raičević, Separation and Purification Technology (2000). [12] L. C. Bell, A. M Posner, J. P Quirk, J. Colloid Interface Sci., 42 2 (1973).

10 20 W. Janusz and E. Skwarek CURRICULA VITAE Władysław Janusz; PhD 1981, Habilitation 2001, Professor Research areas: The main research interest covers various fields of physicochemistry of the solid dispersion (e.g. metal oxides, insoluble carbonates, silicates, phosphates) in the electrolyte solutions especially: origin of the surface charge at the solid /electrolyte interfaces, mechanism of the adsorption of ions at the solid/electrolyte interface, zeta potential measurements of dispersions, structure of the electrical double layer structure at the solid/electrolyte interface, determination of the equilibrium constants of surface hydroxyl groups at the solid/electrolyte interface, specific adsorption of ions at the solid/electrolyte interface, competition of ion adsorption, adsorption of ternary complexes at the solid/electrolyte interface, colloid stability, additionaly radioisotope methods in physical and analytical chemistry. Ewa Skwarek received MSc in organic chemistry, PhD in physical chemistry from the University of Maria Curie Sklodowska in Lublin in 1996 and 2005 respectively. At present, she is a lecturer in Department of Radiochemistry and Colloid Chemistry. Main field of her interests are the adsorption of ions on metal oxides and various minerals, synthesis of adsorbents and structure of the electrical double layer.